1. Domain of the Invention
The present invention relates to the field of radiofrequency transmitters, especially those used in wireless communication terminals such as mobile telephones for example.
The invention relates, more especially, to reconfigurable radiofrequency transmitters which are capable of transmitting over a wide power range and using several transmission formats.
2. Description of the Related Art
In the field of wireless radio communication, there are numerous communication formats which use electromagnetic waves in the radiofrequency band such as the UHF, GSM, EDGE, WCDMA, 3G, etc. standards. Each of these standards is associated with a particular transmission frequency band (one talks of the UHF band, GSM band, 3G band, etc. for example).
Nevertheless, a radio transmitter which is suitable for transmitting in accordance with a particular standard is not generally suitable for transmitting according to another standard. For example, the architecture of a radio transmitter according to the GMSK standard is based on direct modulation and, to achieve this, comprises an analogue modulator followed by a power amplifier. In contrast, polar loop architectures are especially suitable for transmitting non-constant envelope signals such as the 8PSK modulation adopted for the EDGE standard. But the polar loop cannot transmit signals with a bandwidth larger than EDGE signals. On the other hand, only an architecture with direct modulation and a linear amplifier is usually feasible for wideband transmissions such as transmission according to the 3G and OFMD standards.
There are terminals equipped with several transmitters, each of which is dedicated to a particular standard, in order to overcome the incompatibilities between various radiofrequency transmission architectures. Such terminals are therefore bulky and poorly optimised. In addition, the speed of changeover from one type of transmission to another type of transmission is limited, using a terminal of this type for opportunistic use of radiofrequency spectrum is therefore difficult to envisage.
Moreover, terminals which use communication via electromagnetic waves in the radiofrequency band comprise a transmit module which feeds a signal to an antenna, the signal having been generated on the basis of data signals which are, generally speaking, digital.
In terms of performance, these transmit modules need to be analyzed in respect of various parameters. Thus, to the extent that the devices in which they are fitted are frequently portable, one important criterion is their power consumption because this has a direct influence on the device's battery life. Attempts are therefore generally made to achieve low power consumption.
For spectral occupation reasons, signals undergo both phase and amplitude modulation operations before transmission. However, generally speaking, non-constant amplitude signals or signals having a variable envelope impose constraints on the type of amplifier which can be used. In fact, it is preferable to use linear-response amplifiers to amplify non-constant amplitude signals in order to avoid distortion.
Unfortunately, linear amplifiers perform less well in terms of power consumption than non-linear amplifiers.
Moreover, bearing in mind the spectrum packing density of communication systems which share the same frequency band, it is essential to ensure that transmission systems generate extremely little interference in adjacent frequency bands by using highly selective output filtering.
At present, certain techniques have been developed in order to allow the use of non-linear amplifiers to amplify non-constant amplitude signals. A technique known as “outphasing” makes it possible to achieve satisfactory results. This technique involves using a variable-amplitude signal to generate two constant-amplitude signals having a phase difference which varies depending on the amplitude of the original signal.
These two intermediate signals which have a constant envelope and are generated on the basis of the basic signal can thus be amplified by non-linear amplifiers without any risk of introducing signal distortion because the amplitude information no longer exists.
After amplification, these two intermediate signals are added by a recombining stage. Thanks to the opposite phase difference of the two intermediate signals, this recombination makes it possible to obtain a variable-amplitude output signal having the same function as that of the original signal, but after amplification.
This “outphasing” technique, based on theoretical work described by CHIREIX in “High power outphasing modulation”, in Proc IRE, Volume 23 November 1935, pages 1370 to 1392, has been the subject of various improvements, including that described in Document US 2006/0078067.
At present, amplifier modules which use the “outphasing” operating principle use signal recombining stages which employ passive components such as transformers, quarter wavelength transmission lines or magnetic couplers. Such components are inherently relatively bulky and perform poorly in the main and are therefore unsuitable for use in applications in mobile terminals.